Fish are turned off by the smell of algae, study shows

^{A fish in a 'decision chamber.'}
 

Smelling good pays off for coral reefs too. Researchers have discovered that coral larvae and juvenile fish looking to settle down use chemical cues coming from the resident corals and algae on a reef to “smell” their way to a healthy neighborhood and actively avoid reefs in bad shape. This discovery has great implications for coral reef restoration, as the mere closure of degraded reefs to fishing (by turning them into Marine Protected Areas or MPAs) may not be enough to encourage the settling of new fish and corals—particularly if there are enough algae to keep them away outright.

 

This study was conducted by researchers from the Georgia Institute of Technology and Zayed University and published in the journal Science.

 

The larvae of corals and fishes often float and swim with the water currents and travel away from their source reef, settling to the substrate once they find an appropriate spot.

Which way to swim?

The scientists collected water from healthy reefs located inside MPAs (coral cover of 38 to 56%) and fished degraded reefs outside MPAs (coral cover of 4 to 16%, with seaweed cover of 49 to 91%) and continuously flushed them down two-channel flumes. 

 

Larvae from Acropora corals and juvenile damselfishes, butterflyfishes, surgeonfishes, rabbitfishes, wrasses, and cardinalfishes were then placed downstream and observed as to which water source they would swim to.

The coral larvae and juvenile fish consistently chose the water coming from healthy reefs, with juvenile fish having an up to eight-fold increase in preference for water from healthy reefs versus degraded reefs.

The culprit: Sargassum algae

 

To figure out what was attracting or repelling the coral larvae and juvenile fish, the scientists soaked seaweeds, corals, and crustose coralline algae in individual tubs of MPA and non-MPA water to leach out their chemical cues. They discovered that adding Sargassum seaweed algae to MPA water decreased fishes' attraction to the water by up to 86% and corals' attraction by 81%. 

 

In comparison, adding Acropora coral to non-MPA water resulted in a three-fold increase of the water's attractiveness to the fishes. The fishes also vastly preferred the water with sensitive Acropora coral over water with hardy Porites coral. 

 

Sargassum is a tall, brown seaweed that can multiply quickly and take over a reef. The scientists suggested that the presence of sensitive Acropora coral serves as a sign to the fish that they are settling in a healthy reef, as these sensitive corals are among the first to disappear if the reef is stressed. Porites corals are more tolerant to stresses and survive even when Acropora do not.

 

Careful decisions based on smell

 

No-take marine protected areas are often established to serve as a source of coral larvae and juvenile fish for fished reefs. However, "that recruitment may not happen if young fish and coral are not recognizing the degraded area as habitat," said Danielle Dixson, the study's lead author.

 

"Not only are coral smelling good areas versus bad areas, but they're nuanced about it," said Mark Hay, the study's senior author. "They're making careful decisions and can say, 'settle or don't settle'. What this means is that we probably need to manage these reefs in ways that help remove the most negative seaweeds and then help promote the most positive corals."

 

This new research may affect the management schemes of the more than 1,800 MPAs found in the Philippines. Less than 5% of Philippine reefs are in excellent condition (have more than 75% hard coral cover) and MPAs were established to prevent or regulate extractive activities like fishing and to protect the remaining pristine habitats.  — TJD, GMA News
 

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Macy Añonuevo earned her MS Marine Science degree from the University of the Philippines. She is a published science and travel writer and was a finalist in the 2013 World Responsible Tourism Awards under the Best Photography for Responsible Tourism category. Her writings and photographs may be found at www.theislandergirl.com.